In electrophotography an image comprising an electrostatic field pattern, usually of non-uniform strength (also referred to as an electrostatic latent image), is formed on an insulative surface of an electrophotographic element comprising at least a photoconductive layer and an electrically conductive substrate. The electrostatic latent image is usually formed by imagewise radiation-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on the insulative surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
In latent image formation the imagewise radiation-induced dissipation of the initially uniform electrostatic field is brought about by the creation of electron/hole pairs, which are generated by a material, often referred to as a photoconductive or charge-generation material, in the electrophotographic element in response to exposure to imagewise actinic radiation. Depending upon the polarity of the initially uniform electrostatic field and the types of materials included in the electrophotographic element, part of the charge that has been generated, i.e., either the holes or the electrons, migrates toward the charged insulative surface of the element in the exposed areas and thereby causes the imagewise dissipation of the initial field. What remains is a non-uniform field constituting the electrostatic latent image.
Several types of electrophotographic recording elements are known for use in electrophotography. In many conventional elements, the active photoconductive or charge-generation materials are contained in a single layer. This layer is coated on a suitable electrically conductive support or on a non-conductive support that is overcoated with an electrically conductive layer. In addition to single-active-layer electrophotographic recording elements, various multi-active electrophotographic recording elements are known. Such elements are sometimes called multi-layer or multi-active-layer elements because they contain at least two active layers that interact to form an electrostatic latent image.
A class of photoconductive materials useful in the aforementioned single-active-layer and multiactive elements is the class of perylene pigments, particularly perylene-3,4,9,10-tetracarboxylic acid imide derivatives. Representative examples of patents pertaining to such perylene photoconductive pigments include, U.S. Pat. No. 4,578,334, issued Mar. 25, 1986, which describes multi-active electrophotographic recording elements that contain, as photoconductive materials, certain crystalline forms of N,N'-bis(2-phenethyl)perylene-3,4:9,10-bis(dicarboximide) characterized by particular spectral absorption and x-ray diffraction characteristics; U.S. Pat. No. 4,714,666, issued Dec. 22, 1987, which describes single-active-layer electrophotographic elements and multi-active elements containing, as photoconductive materials, asymmetrically substituted perylene-3,4,9,10-tetracarboxylic acid imide derivatives, and U.S. Pat. No. 4,792,508, issued Dec. 20, 1988, which describes multi-active elements that contain as photoconductive materials, mixtures of cis- and trans-naphthimidazole perylenes.
Unfortunately, electrophotographic recording elements of the prior art which contain photoconductive perylene materials have typically suffered from one or more disadvantages that have significantly restricted their use. For example, vacuum sublimation is frequently required to deposit photoconductive perylene pigments in a crystal form suitable for high speed electrophotographic elements. Thus, U.S. Pat. No. 4,578,334 describes a process wherein a perylene pigment is deposited by vacuum sublimation in the form of an amorphous layer and is thereafter converted to the photoconductive crystalline form by contacting the layer with an appropriate liquid composition. Vacuum sublimation, however, is a batch process which makes production scale runs quite costly and thin sublimed films are fragile and susceptible to damage until they can be protected by a more durable overcoat.
To avoid the disadvantages inherent in forming photoconductive perylene pigment layers using vacuum sublimation techniques and the fragile nature of such layers; electrophotographic layers have been coated from liquid coating compositions comprising finely-divided photoconductive perylene pigments in solvent solutions of polymeric binders, as described, for example, in U.S. Pat. No. 4,714,666. To achieve acceptable electrophotographic speed with such a coating it is necessary that the perylene pigment be in a form (crystalline or amorphous) that is highly photoconductive and sufficiently and stably dispersed in the coating composition to permit it to be applied at a low enough concentration to form a very thin layer having high electrophotographic speed. Forming such photoconductive perylene pigments and dispersing the pigment particles to the necessary degree is extremely difficult. Thus, forming highly stable dispersions of photoconductive perylene pigments in liquid coating compositions is not easily achieved with conventional procedures. Such conventional procedures normally involve simply mixing the components of a liquid coating composition, e.g., a dispersion of photoconductive perylene pigment in a solvent solution of polymeric binder, in a suitable mixing device such as a ball mill or a paint shaker. Unfortunately, such procedures do not adequately disperse the pigment particles and frequently particle agglomerates are formed in the coated layers. Such agglomerates detrimentally affect the image quality of copies formed with electrophotographic elements containing such layers. Furthermore, prolonged mixing of the photoconductive perylene pigment in a device such as a ball mill can damage the pigment structurally so that electrophotographic performance is detrimentally affected.
From the foregoing discussion, it is evident that a method that is capable of providing finely-divided photoconductive perylene pigments that have excellent sensitometric characteristics and form stable dispersions would represent a significant advance in the art. Likewise, it is evident that a method for preparing electrophotographic coating compositions that have finely-divided photoconductive perylene pigments dispersed in a solvent solution of polymeric binder and can be used to form high speed electrophotographic layers without requiring sublimation coating techniques would also represent such an advance. It is an objective of this invention to provide a novel method that will achieve such advances in the art.